The present invention relates to a metering apparatus for introducing a powdery medium into a fluid. The invention also relates to a mixing plant having such a metering apparatus for mixing a drilling fluid, as well as to a method for introducing a powdery medium into a fluid.
The following discussion of related art is provided to assist the reader in understanding the advantages of the invention, and is not to be construed as an admission that this related art is prior art to this invention.
A drilling fluid is typically employed for supporting the drill feed when constructing drill holes in the ground, in particular horizontal drill holes. The drilling fluid is used to soften the ground in advance of the drill head of the drilling apparatus to improve the cutting performance of the drill head. The drilling fluid can also be used to lubricate the drill head and the drill rod, which is rotatably driven in the drill hole, so as to reduce friction with the ground. In addition, the drilling fluid can be used to flush out the soil removed by the drill head through the annular gap between the drill rod and the wall of the drill hole or through an annular gap of dual drill rods.
The drilling fluid is typically a mixture of water and bentonite, and sometimes several additives. Bentonite is a mixture of different clay materials, with the largest component being montmorillonite (generally with a content of 60% to 80%). Additional accompanying materials may be quartz, mica, feldspar, pyrite and sometimes also calcite. Due to the montmorillonite content, bentonite has strong water absorption and swelling capability.
Water into which bentonite has been stirred can have thixotropic characteristics, so that it behaves like a fluid when in motion, but like a solid structure when at rest. Because of this behavior, a drilling fluid composed of water and bentonite can also be used for supporting the wall of the drill hole, thereby preventing a collapse.
The introduction of bentonite into water poses a particular challenge, because the bentonite has the tendency to lump together in contact with water. The drilling fluid is typically stirred in large storage vessels with dynamic mixing devices and thereafter transported in batches to the construction site where the drilling fluid is to be used. However, such batch-wise mixing is quite cumbersome. In addition, after the drill hole has been completed, the unused portion of the last batch must be disposed of, which is complex and expensive.
Bentonite can also be introduced directly in the water in the region of a high-pressure pump, which is provided for transporting the drilling fluid through the drill rod to the drill head of a horizontal drilling apparatus, in order to take advantage of the turbulences produced in the water by the high-pressure pump for mixing the bentonite with the water. A swelling section can be arranged downstream of the high-pressure pump, where the bentonite-water-mixture is given time to swell before it is transported through the drill rod to the drill head.
It would therefore be desirable and advantageous to obviate prior art shortcomings and to provide metering device for introducing a powdery medium into a fluid, whereby problems associated with lumping of the powdery medium upon contact with the fluid can be reduced or even eliminated. It would also be desirable to provide a corresponding method and a mixing plant for mixing a drilling fluid.